#17664: {% if %} template tag silences exceptions inconsistently
-------------------------------------+-------------------------------------
Reporter: Tai Lee | Owner: Robert
| Roskam
Type: Bug | Status: assigned
Component: Template system | Version: master
Severity: Normal | Resolution:
Keywords: smart if tag | Triage Stage: Accepted
queryset exception silenced |
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by Robert Roskam):
So I've figured out what's going on here. We have 2 distant parts
interacting here:
(1) the templatetag if parser
(https://github.com/django/django/blob/master/django/template/smartif.py#L174)
(2) the lookup_resolver
(https://github.com/django/django/blob/master/django/template/base.py#L819)
Template literals with no booleans operations are passed down through the
if parser in (1) and no evaluation is down on them. All the capturing of
values and forcing them to False is done in the infix and prefix functions
of (1). Since both of these 2 previous things are true, any exception that
is raised will be caught if it uses any boolean logic, so given that:
{{{
def _raise_exception():
raise Exception
a = _raise_exception
}}}
All of these will raises exceptions:
{{{
{% if a %}
{% if a|length %}
{% if a||add:"2" %}
}}}
However, all of these will coerce `a` to False:
{{{
{% if a == 1 %}
{% if a and a%}
{% if not not a %}
}}}
As best as I can tell, these are being totally consistent with their
design intent. That is to say, they're operating exactly like the docs say
they should. If we don't like the fact that {{{a}}} raises an error and
{{{not a}}} doesn't, then I think we should make a deliberate design
decision to do one or the other.
--
Ticket URL: <https://code.djangoproject.com/ticket/17664#comment:21>
Django <https://code.djangoproject.com/>
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